# Counter\_cache to calculate child entries

**URL:** <https://www.ruby-forum.com/t/counter-cache-to-calculate-child-entries/73246>\
**Category:** Rails\
**Created:** [October 23, 2006, 8:02am UTC](https://www.ruby-forum.com/t/counter-cache-to-calculate-child-entries/73246 "2006-10-23T08:02:32Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![roger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/c0e974/32.png) [@roger](https://www.ruby-forum.com/u/roger)\
**Post date:** [October 23, 2006, 8:02am UTC](https://www.ruby-forum.com/t/counter-cache-to-calculate-child-entries/73246/1 "2006-10-23T08:02:32Z")

</div>

The counter cache method can be used to store the number of child items  
with ease.  
My scheme of things has multiple users adding entries into the child  
table. Will the active record handle this? Will the number be updated  
accordingly if more than one user simultaneously add child entries?

---

<div class="post-metadata">

**Author:** ![roger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/c0e974/32.png) [@roger](https://www.ruby-forum.com/u/roger)\
**Post date:** [October 24, 2006, 1:41am UTC](https://www.ruby-forum.com/t/counter-cache-to-calculate-child-entries/73246/2 "2006-10-24T01:41:41Z")

</div>

Roger wrote:

> The counter cache method can be used to store the number of child items  
> with ease.  
> My scheme of things has multiple users adding entries into the child  
> table. Will the active record handle this? Will the number be updated  
> accordingly if more than one user simultaneously add child entries?

No, the counter cache can be set incorrectly if children are added  
concurrently. To be sure the counter\_cache is updated correctly,  
code like:

Parent.transaction do  
@parent = Parent.find( …, :from =\> ‘parents for update’ )  
@parent.children.create( params[:child] )  
end

Similarly for child deletion.

–  
We develop, watch us RoR, in numbers too big to ignore.

---

<div class="post-metadata">

**Author:** ![roger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/c0e974/32.png) [@roger](https://www.ruby-forum.com/u/roger)\
**Post date:** [October 24, 2006, 3:19am UTC](https://www.ruby-forum.com/t/counter-cache-to-calculate-child-entries/73246/3 "2006-10-24T03:19:47Z")

</div>

Mark Reginald J. wrote:

> No, the counter cache can be set incorrectly if children are added  
> concurrently. To be sure the counter\_cache is updated correctly,  
> code like:
> 
> Parent.transaction do  
> @parent = Parent.find( …, :from =\> ‘parents for update’ )  
> @parent.children.create( params[:child] )  
> end

That said, as long as you wrap the child creation in a transaction  
(if Rails has not already done so internally, as is the case  
for creation via the parent association), you’re pretty right  
because counter cache updates are done as single-statement updates  
in after\_create.

It depends on whether you just want your child count to stay correct,  
not caring if you read a slightly stale count on occasion, or whether  
you want the correct count every time you read it.

–  
We develop, watch us RoR, in numbers too big to ignore.

---

<div class="post-metadata">

**Author:** ![roger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/c0e974/32.png) [@roger](https://www.ruby-forum.com/u/roger)\
**Post date:** [October 24, 2006, 4:19am UTC](https://www.ruby-forum.com/t/counter-cache-to-calculate-child-entries/73246/4 "2006-10-24T04:19:51Z")

</div>

Roger wrote:

> The counter cache method can be used to store the number of child items  
> with ease.  
> My scheme of things has multiple users adding entries into the child  
> table. Will the active record handle this? Will the number be updated  
> accordingly if more than one user simultaneously add child entries?

On third thoughts:

Yes, the counter cache will correctly handle concurrent child insertions  
and removals, but any updates you do to parent objects should be done  
in a “select…for update” transaction because the counter cache may be  
updated by another request while the object is being found, updated, and  
saved.

Really Active Record should be changed so it leaves out counter  
cache attributes when doing update saves.

–  
We develop, watch us RoR, in numbers too big to ignore.